As described quite thoroughly on Pages 166 through 169 of the Betz Handbook of Industrial Water Conditioning, 6th Edition, 1962, Betz Laboratories, Inc., Trevose, Pa., the control of dissolved oxygen in water systems, particularly boiler water or more generally steam producing systems, is a must because of its capacity to promote the corrosion of metallic parts in contact with the water.
Dissolved oxygen can be introduced into the system not only in the makeup water but also due to air infiltration of the condensate system. When dissolved oxygen is present in the feedwater, an attack of the feed line, closed heaters and economizer can be expected with the severity of the problem dependent on the concentration of dissolved oxygen and the temperature involved. One of the most serious aspects of oxygen corrosion is that it generally occurs as pitting so that the attack is concentrated in a small area of the total metal surface. With this type of corrosion, failures can occur even though only a relatively small portion of the metal has been lost.
The influence of temperature on the corrosivity of dissolved oxygen is particularly important in such equipment as closed heaters and economizers where the water temperature is increased very rapidly. Under such conditions, an additional driving force for the oxidation reaction is present and for this reason, even very small quantities of dissolved oxygen in feedwater can cause severe corrosion in such equipment.
When oxygen is present in the feedwater entering the boiler, a portion will be flashed and will leave the boiler with the steam. The remainder of the dissolved oxygen can attack the boiler metal. While the point of attack will vary with the boiler design and feedwater distribution, oxygen pitting is usually concentrated adjacent to the water level in the feedwater drum.
The first and most important step in eliminating the corrosive influence of dissolved oxygen is mechanical deaeration of the boiler feedwater. Efficient deaeration will reduce the dissolved oxygen content of the boiler feedwater to a very low value. It is advisable to follow mechanical deaeration by chemical deaeration in order to remove the last traces of dissolved oxygen. Where mechanical deaeration is not employed, chemical deaeration must be used for the removal of the entire oxygen content of the feedwater.
Sodium sulfite and sodium bisulfite are the chemical agents most commonly employed for chemical deaeration due to their low cost, ease of handling and their lack of scale forming properties. The oxygen scavenging characteristics of sodium sulfite are illustrated by the following reaction: EQU 2Na.sub.2 SO.sub.3 +O.sub.2 =2Na.sub.2 SO.sub.4 EQU (sodium sulfite+oxygen=sodium sulfate)
The reaction with sodium bisulfite is of course quite similar,
The removal of 1.0 ppm dissolved oxygen theoretically requires 7.88 ppm of chemically pure sodium sulfite. However, use of a technical grade of sodium sulfite or bisulfite combined with handling and blowdown losses as encountered in actual plant operation usually requires the feed of approximately 10 pounds of sodium sulfite or bisulfite for each pound of oxygen. Requirements will also depend on the concentration of excess sulfite maintained in the boiler water.
To assure complete oxygen removal, it is necessary to maintain a residual concentration of sulfite in the boiler water. The residual required depends on a number of factors such as the method of feed and the point of application, the dissolved oxygen concentration and the variation in the dissolved oxygen concentration of the feedwater.
Continuous feed of the sodium sulfites is generally required for complete oxygen removal. In the majority of plants, the most suitable point of application is the storage compartment of the deaerating or open heater. In other plants, sufficient reaction time will be allowed with application to the suction side of the boiler feed pump. While intermittent application is generally not recommended, it has been found in some low pressure systems that adequate protection is provided as long as the additions of sodium sulfite are made with sufficient frequency to continuously maintain the proper residual concentration in the boiler water.
Testing of the boiler water for sulfite residual and recording the quantity of sulfite required serves also as a quick check on heater deaeration efficiency in those plants where the oxygen content of the feedwater is not determined regularly. Any decrease in boiler water sulfite residual, and consequent need for increased feed of the sodium sulfites, is an indication that heater operation should be checked to ascertain and correct the reason for increased oxygen content of the boiler feedwater.
The speed of the sulfite-oxygen reaction is affected by a number of factors, the most important being temperature. The reaction time decreases with increased temperature. In general, the reaction speed doubles for every 10.degree. C. increase in temperature. At temperatures of 212.degree. F. and above the reaction is quite rapid. It has also been found that the presence of an excess or overfeed of the sodium sulfites will increase the reaction rate. Several investigators have shown that the reaction proceeds most rapidly at pH values in the vicinity of 9.0-10.0.
Research directed toward increasing the speed of the oxygen-sulfite reaction has determined that certain water-soluble materials act as catalysts in speeding this reaction to completion. The most suitable catalysts are the heavy metal cations of two or more valences. Iron, copper, cobalt, nickel and manganese are among the more effective catalytic aids to the oxygen-sulfite reaction. Combinations of several of these heavy metal cations have proved effective in providing a continuously active influence on the speed of reaction. The catalysts are introduced as their water-soluble salts, i.e., chloride, sulfate, nitrate, etc.
As a result of research on catalytic aids for oxygen removal, catalyzed sodium sulfite and sodium bisulfite formulations were developed. Through the incorporation of suitable catalysts and the sodium sulfites in one formulation, a material was available which would consistently provide practically instantaneous oxygen removal, even when the water possesses natural inhibitory properties. The concentration of the catalyst added is dependent upon the sulfite concentration in the solution. Concentrations of the catalyst of 0.05 to 1.0% by weight of the weight of sulfite present have been found to be effective. Most commonly, the weight used is approximately 0.1%.
Catalyzed sodium sulfite or bisulfite is used in low temperature systems for oxygen removal and also finds application in boiler systems where the feedwater temperature is low, where mechanical deaeration is not complete or where it is essential to obtain rapid reaction for prevention of pitting in feed lines, closed heaters and economizers.
As indicated in the foregoing discussion, the use of catalysts in conjunction with the sodium sulfite and bisulfite has proven quite effective. However, there is a problem associated with aqueous solutions of these products, particularly aqueous solutions of sodium bisulfite, which occurs during storage in storage tanks. Unusual as it may seem, the problem did not occur when the product was contained for example in drums. It was discovered that at several locations having an aqueous solution containing 33% sodium bisulfite and 0.1% cobalt chloride catalyst (based on weight of bisulfite) stored in large bulk tanks the solution contained a reddish brown sludge which resulted in clogged feed lines and pumps, causing shutdowns.
Samples of the reddish brown sludge were analyzed and found to be composed of cobalt sulfite. Retained samples of the solution showed no evidence of any precipitation even in samples over two years old.
It was accordingly concluded that the products evidencing precipitation had been subjected to conditions during bulk storage which promoted the instability of the sodium bisulfite/cobalt chloride solution.
Since the precipitation of CoSO.sub.3 occurred only in vented bulk storage tanks, it was assumed that the loss of sulfur dioxide gas from the cobalt catalyzed bisulfite solution was a critical factor in the precipitation. The decrease in the concentration of NaHSO.sub.3 in the complaint samples could be attributed to the evolution of sulfur dioxide gas. This explained the pH rise in the sludged samples, since bisulfite solutions evolve sulfur dioxide according to the reaction: EQU 2HSO.sub.3 -.fwdarw.SO.sub.2 .uparw.+H.sub.2 O+SO.sub.3 =
The net reaction results in the loss of two bisulfite protons to water and the formation of a sulfite ion. The pH and sulfite ion concentration of an open air bisulfite solution rises as sulfur dioxide is evolved. The increase in the sulfite ion concentration accompanying the evolution of SO.sub.2 gas leads to formation of cobalt sulfite.
This mechanism was proved in the laboratory by dividing a sodium bisulfite/cobalt chloride solution into two jars. One jar was vented with a small hole in the lid while the other was tightly sealed. The starting pH was 3.2. Over a period of one month the sealed jar maintained its pH of 3.2. The pH of the solution in the vented jar rose to 5.1 and a red precipitate of cobalt sulfite formed on the bottom of the jar. The sulfite ion concentration of a bisulfite solution was also increased by addition of enough caustic to raise the pH from 3.8 to 5.0. The precipitation of cobalt sulfite occurred overnight.
Applicants considered, but quickly eliminated, solutions to the problem proposed including mechanical adjustments to the tanks, addition of acid in the field to maintain a low pH, and replacement of the catalyst with one more stable under higher pH.